1 / 23

Block Ciphers

Block Ciphers. Block Ciphers. Modern version of a codebook cipher In effect, a block cipher algorithm yields a huge number of codebooks Specific codebook determined by key It is OK to use same key for a while Just like classic codebook Initialization vector (IV) is like additive

florencel
Download Presentation

Block Ciphers

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Block Ciphers Block Ciphers 1

  2. Block Ciphers • Modern version of a codebook cipher • In effect, a block cipher algorithm yields a huge number of codebooks • Specific codebook determined by key • It is OK to use same key for a while • Just like classic codebook • Initialization vector (IV) is like additive • Change the key, get a new codebook Block Ciphers 2

  3. (Iterated) Block Cipher • Plaintext and ciphertext “units” are fixed sized blocks • Typical block sizes: 64 to 256 bits • Ciphertext obtained from plaintext by iterating a round function • Input to round function consists of key and the output of previous round • Most are designed for software Block Ciphers 3

  4. Multiple Blocks • How to encrypt multiple blocks? • A new key for each block? • As bad as (or worse than) a one-time pad! • Encrypt each block independently? • Make encryption depend on previous block(s), i.e., “chain” the blocks together? • How to handle partial blocks? Block Ciphers 4

  5. Block Cipher Modes • We discuss 3 (many others) • Electronic Codebook (ECB) mode • Encrypt each block independently • There is a serious weakness • Cipher Block Chaining (CBC) mode • Chain the blocks together • Better than ECB, virtually no extra work • Counter Mode (CTR) mode • Like a stream cipher (random access) Block Ciphers 5

  6. ECB Mode • Notation: C = E(P,K) and P = D(C,K) • Given plaintext P0,P1,…,Pm,… • Obvious way to use a block cipher is EncryptDecrypt C0 = E(P0, K), P0 = D(C0, K), C1 = E(P1, K), P1 = D(C1, K), C2 = E(P2, K),… P2 = D(C2, K),… • For a fixed key K, this is an electronic version of a codebook cipher (no additive) • A new codebook for each key Block Ciphers 6

  7. ECB Cut and Paste Attack • Suppose plaintext is Alice digs Bob. Trudy digs Tom. • Assuming 64-bit blocks and 8-bit ASCII: P0 = “Alice di”, P1 = “gs Bob. ”, P2 = “Trudy di”, P3 = “gs Tom. ” • Ciphertext: C0,C1,C2,C3 • Trudy cuts and pastes: C0,C3,C2,C1 • Decrypts as Alice digs Tom. Trudy digs Bob. Block Ciphers 7

  8. ECB Weakness • Suppose Pi = Pj • Then Ci = Cj and Trudy knows Pi = Pj • This gives Trudy some information, even if she does not know Pi or Pj • Trudy might know Pi • Is this a serious issue? Block Ciphers 8

  9. Alice Hates ECB Mode • Alice’s uncompressed image, Alice ECB encrypted (TEA) • Why does this happen? • Same plaintext block  same ciphertext! Block Ciphers 9

  10. CBC Mode • Blocks are “chained” together • A random initialization vector, or IV, is required to initialize CBC mode • IV is random, but need not be secret EncryptionDecryption C0 = E(IV  P0, K), P0 = IV  D(C0, K), C1 = E(C0  P1, K), P1 = C0  D(C1, K), C2 = E(C1  P2, K),… P2 = C1  D(C2, K),… Block Ciphers 10

  11. CBC Mode • Identical plaintext blocks yield different ciphertext blocks • Cut and paste is still possible, but more complex (and will cause garbles) • If C1 is garbled to, say, G then P1  C0  D(G, K), P2  G  D(C2, K) • But P3 = C2  D(C3, K), P4 = C3  D(C4, K),… • Automatically recovers from errors! Block Ciphers 11

  12. Alice Likes CBC Mode • Alice’s uncompressed image, Alice CBC encrypted (TEA) • Why does this happen? • Same plaintext yields different ciphertext! Block Ciphers 12

  13. Counter Mode (CTR) • CTR is popular for random access • Use block cipher like stream cipher EncryptionDecryption C0 = P0  E(IV, K), P0 = C0  E(IV, K), C1 = P1  E(IV+1, K), P1 = C1  E(IV+1, K), C2 = P2  E(IV+2, K),… P2 = C2  E(IV+2, K),… • CBC can also be used for random access!!! Block Ciphers 13

  14. Integrity Block Ciphers 14

  15. Data Integrity • Integrity prevent (or at least detect) unauthorized modification of data • Example: Inter-bank fund transfers • Confidentiality is nice, but integrity is critical • Encryption provides confidentiality (prevents unauthorized disclosure) • Encryption alone does not assure integrity (recall one-time pad and attack on ECB) Block Ciphers 15

  16. MAC • Message Authentication Code (MAC) • Used for data integrity • Integrity not the same as confidentiality • MAC is computed as CBC residue • Compute CBC encryption, but only save the final ciphertext block Block Ciphers 16

  17. MAC Computation • MAC computation (assuming N blocks) C0 = E(IV  P0, K), C1 = E(C0  P1, K), C2 = E(C1  P2, K),… CN1 = E(CN2  PN1, K) = MAC • MAC sent along with plaintext • Receiver does same computation and verifies that result agrees with MAC • Receiver must also know the key K Block Ciphers 17

  18. Why does a MAC work? • Suppose Alice computes C0 = E(IVP0,K), C1 = E(C0P1,K), C2 = E(C1P2,K), C3 = E(C2P3,K) = MAC • Alice sends IV,P0,P1,P2,P3 and MAC to Bob • Trudy changes P1 to X • Bob computes C0 = E(IVP0,K), C1= E(C0X,K), C2= E(C1P2,K), C3= E(C2P3,K) = MAC MAC • Propagates into MAC (unlike CBC decryption) • Trudy can’t change MAC to MAC without K Block Ciphers 18

  19. Confidentiality and Integrity • Encrypt with one key, MAC with another • Why not use the same key? • Send last encrypted block (MAC) twice? • Can’t add any security! • Use different keys to encrypt and compute MAC; it’s OK if keys are related • But still twice as much work as encryption alone • Confidentiality and integrity with one “encryption” is a research topic Block Ciphers 19

  20. Uses for Symmetric Crypto • Confidentiality • Transmitting data over insecure channel • Secure storage on insecure media • Integrity (MAC) • Authentication protocols (later…) • Anything you can do with a hash function (upcoming chapter…) Block Ciphers 20

  21. Feistel Cipher • Feistel cipher refers to a type of block cipher design, not a specific cipher • Split plaintext block into left and right halves: Plaintext = (L0,R0) • For each round i=1,2,...,n, compute Li= Ri1 Ri= Li1 F(Ri1,Ki) where F is round functionand Ki is subkey • Ciphertext = (Ln,Rn) Block Ciphers 21

  22. Feistel Cipher • Decryption: Ciphertext = (Ln,Rn) • For each round i=n,n1,…,1, compute Ri1 = Li Li1 = Ri F(Ri1,Ki) where F is round functionand Ki is subkey • Plaintext = (L0,R0) • Formula “works” for any function F • But only secure for certain functions F Block Ciphers 22

  23. Conclusions • Block ciphers widely used today • Fast in software, very flexible, etc. • Not hard to design strong block cipher • Tricky to make it fast and secure • Next: Hellman’s TMTO • Then: CMEA, Akelarre and FEAL Block Ciphers 23

More Related